This invention relates to the field of machine vision, and more specifically to a method and apparatus of generating a pattern of projected light for obtaining three-dimensional inspection data for manufactured parts in a manufacturing environment.
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There is a widespread need for inspection data for electronic parts in a manufacturing environment. One common inspection method uses a video camera to acquire two-dimensional images of a device-under-test.
Height distribution of a surface can be obtained by projecting a light-stripe pattern onto the surface and then re-imaging the light pattern that appears on the surface.
One technique for extracting this information based on taking multiple images (three or more) of the light pattern that appears on the surface while shifting the position (phase) of the projected light stripe pattern is referred to as phase shifting interferometry, as disclosed in U.S. Pat. Nos. 4,641,972 and 4,212,073 (incorporated herein by reference).
The multiple images are usually taken using a CCD (charge-coupled device) video camera with the images being digitized and transferred to a computer where phase-shift analysis, based on images being used as xe2x80x9cbuckets,xe2x80x9d converts the information to a contour map (i.e., a three-dimensional representation) of the surface.
The techniques used to obtain the multiple images are based on methods that keep the camera and viewed surface stationary with respect to each other while moving the projected pattern.
One technique for capturing just one bucket image using a line scan camera is described in U.S. Pat. No. 4,965,665 (incorporated herein by reference).
U.S. Pat. Nos. 5,398,113 and 5,355,221 (incorporated herein by reference) disclose white-light interferometry systems which profile surfaces of objects.
In U.S. Pat. No. 5,636,025 (incorporated herein by reference), an optical measuring system is disclosed which includes a light source, gratings, lenses, and camera. A mechanical translation device moves one of the gratings in a plane parallel to a reference surface to effect a phase shift of a projected image of the grating on the contoured surface to be measured. A second mechanical translation device moves one of the lenses to effect a change in the contour interval. A first phase of the points on the contoured surface is taken, via a four-bucket algorithm, at a first contour interval. A second phase of the points is taken at a second contour interval. A control system, including a computer, determines a coarse measurement using the difference between the first and second phases. The control system further determines a fine measurement using either the first or second phase. The displacement or distance, relative to the reference plane, of each point is determined, via the control system, using the fine and coarse measurements.
Current vision inspection systems have many problems. Among the problems are assorted problems associated generating precise, high-contrast Moire patterns for scanning Moire interferometry (SMI) measurements. Conventional systems have used 50% duty-cycle black-and-clear stripes, counting on diffraction and poor focus to blur the pattern into an approximately sine-wave pattern. Additional precision was obtained by generating a lined pattern, where each line along its edges increased in density, to obtain a sine-wave pattern, such as described in U.S. patent application Ser. No. 09/350,037, entitled xe2x80x9cMACHINE-VISION SYSTEM AND METHOD HAVING A SINE-WAVE PROJECTION PATTERNxe2x80x9d, filed Jul. 8, 1999 and incorporated by reference. Such patterns are difficult and costly to generate with precision. They also typically have less contrast and less light transmitted in the pattern than is desired.
To overcome the problems stated above as well as other problems, there is a need for an improved machine-vision system and more specifically for a mechanical apparatus and method for inspecting manufactured parts such as disk-drive suspensions, and for determining various heights, dimensions, and angles of the parts.
In the context of a machine-vision system for inspecting a part, this invention includes method and apparatus to provide high-speed 3D (three-dimensional) inspection of manufactured parts. A binary pattern of opaque and clear stripes of different widths is used to generate a pattern that, at the object, approximates a sine-wave pattern to excellent precision and accuracy.
The invention provides a method, a grating, and a 3D imaging system using a binary grating in the projection system to generate patterned light. The grating includes a binary grating having a cycle pattern, wherein each cycle includes a first substantially clear stripe of a first width and a first substantially opaque stripe of substantially the same first width, and between the first substantially clear stripe and the first substantially opaque stripe are a second substantially opaque stripe of a second width narrower than the first width, and a second substantially clear stripe of substantially the second width. In some embodiments, a plurality of alternating narrow clear and opaque stripes are positioned between the relatively wider first substantially clear stripe and first substantially opaque stripe, in order that the optics of the projection optical system and the relative motion between the projection light pattern and the imaging time (trapezoidal integration) smooth the resulting pattern to provide a linear sine-wave-like pattern of projection light.